The present invention relates to a fuel assembly for a nuclear reactor of pressurized-water type. Such a fuel assembly of a known design comprises a plurality of fuel rods held together into a bundle by means of spacers arranged along the fuel rods as well as guide tubes arranged between the fuel rods and fixed to the spacers. The bundle is arranged between a top nozzle and a bottom nozzle. These are provided with a plurality of openings for the coolant flow which is to pass through the fuel assembly. The fuel assembly has no surrounding casing at all, a so-called fuel box, with which the fuel assemblies in a boiling-type nuclear reactor are provided.
It is further known to throttle the coolant flow to the most burnt-up fuel assemblies in a nuclear reactor to be able, in the case of an unchanged total coolant flow through the reactor core, to increase the coolant flow to the less burnt-up fuel assemblies and hence to be able to absorb more power from these. The throttling normally takes place at the openings of the bottom and/or top nozzle, either manually during refuelling or by some form of automatic means. This type of control is effective for fuel assemblies provided with fuel boxes but less effective in the case of boxless fuel assemblies for a pressurized-water reactor. This is due to the fact that the coolant flow may divert in the lateral direction over to adjacently positioned fuel assemblies. Since particularly in highly loaded fuel assemblies there is a tendency for steam formation on the fuel rods and thus higher resistance to the passage of the coolant flow than in fuel assemblies subjected to lower load, the coolant flow has a tendency to pass from fuel assemblies subjected to higher load to those subjected to lower load even at a relatively short distance from the bottom nozzles of the assemblies when throttling the coolant flow in the fuel assemblies subjected to lower load.